In the buccal ganglia of Aplysia, presynaptic neurons B4 and B5 produce similar inhibitory postsynaptic currents (PSCs) in several postsynaptic follower cells. Previous work has shown that both duration and amplitude of these PSCs vary, that each parameter may be altered transiently by manipulating presynaptic activity, and that these variations affect synaptic efficacy. 2. To permit synapse-to-synapse comparisons, the mean and coefficient of variation (CV) of both peak conductance (gpeak) and time constant of decay (tau) were determined for sets of synaptic currents evoked by direct intracellular stimulation of presynaptic neurons. For 56 synapses, gpeak = 0.40 +/- 0.33 (SD) microS for a CV of 0.83, and tau = 19.7 +/- 4.0 ms for a CV of 0.20. The synapse-to-synapse variability was within 5% of values obtained from a previous population. 3. The relative contributions of presynaptic and postsynaptic factors to efficacy and variability of PSCs were examined by recordings from two classes of three-cell networks and by comparing values of gpeak and tau at synapses sharing a common presynaptic or postsynaptic neuron. 4. In the first case, paired presynaptic inputs from B4 and B5 converged on a common postsynaptic cell. For 16 examples of this case, mean values of both gpeak and tau recorded in a single follower cell, but produced by different presynaptic neurons, were significantly closer than those recorded in different followers (P less than 0.001). The common postsynaptic cell did not constrain variability of these parameters; CVs for paired synapses were not significantly different from the population (P greater than 0.1).(ABSTRACT TRUNCATED AT 250 WORDS)
